Implement workarounds for servers not implementing text search and servers not implementing uid search

[tobixen]

As with explicit substring search (search.py, search for "special compatibility-case for servers that do not support substring search"), if the server does not support text search at all, everything should be fetched and client-side filtering should be done.

This is quite trivial, but:

• The search.py should probably be refactored a bit first, I hate code duplication.
• Does there exist servers that supports search by uid but does not support text search? If so, make a special case for searching by uid
• If not, then the compatibility feature "search.text.by-uid" can be removed